1. Field of the Invention
The present invention relates to a method of manufacturing a diffraction grating. Such a diffraction grating may be used, for example, in an optical apparatus such as a spectroscope.
2. Description of the Related Art
A phase diffraction grating is a diffraction grating made so that a phase difference is given to incident light in one cycle of grating by making the shape of a grating groove into a blazed shape, a sinusoidal shape, or a rectangular wave shape so that a diffraction light intensity property can be selected according to its use. A blazed diffraction grating is a phase diffraction grating having a grating groove with a triangular cross-section, and is manufactured by sequentially forming a ruling, with a grating groove of a predetermined cross-sectional shape, by a cutting process using a grind stone or a shaving process using a diamond cutting tool.
A blazed diffraction grating made of a crystalline material is a diffraction grating designed to use high-order diffraction light. In order to obtain good optical properties, it is necessary that the machining in which the grating groove is formed is performed in a ductile mode. A critical cutting thickness determines a boundary between the ductile mode and a brittle mode. It is known that the critical cutting thickness depends on a relation between a crystal orientation of a slip surface and a worked surface, a relation between a crystal axis and a cutting direction, and a relation between the crystal axis and a cutting force direction (which depends on a tool rake angle and the cutting direction) in a crystal. For example, Japanese Patent Application Laid-Open No. 2003-075622 discusses that the grating groove is formed by using a groove processing surface (a surface on which the grating groove is to be formed) as a (111) plane, using a cutting direction as a (111) plane in a crystalline material made of Si or Ge, and using a single crystal diamond cutting tool in an ultra-precision cutting process.
However, a stable ductile mode is not obtained even when the crystal orientation (plane direction of crystal) of the groove processing surface (diffraction grating plain surface) and the critical cutting thickness are used as parameters. The blazed diffraction grating is influenced by at least one of the long side and the short side that constitute the grating groove when the blazed diffraction grating is used as a transmission type or an immersion type (backside reflection type). However, it is common to use the blazed diffraction grating so that the incident direction of light is perpendicular to the short side of the cross-sectional shape of the grating. The surface roughness (profile irregularity) of the short side is important in the case of a reflection diffraction grating.
The crystalline materials made of gallium phosphide (GaP) or gallium arsenide (GaAs) are used as optical materials, but the machining in which the grating groove of a blazed diffraction grating is formed from these crystalline materials has not been proposed.